The invention concerns a method of measuring the permeability of a coating on a plastic wall to low-molecular gases, for example oxygen, wherein the diffusion coefficient and/or solubility coefficient of the coating with respect to the low-molecular gas is approximately an order of magnitude lower than that of the plastic wall, in which the space on the coating side is subjected to the action of a first fluid and is flushed through until substantially only said first fluid is in said space.
It is known for fluid packs to be made from plastic material, for example in the form of bottles with an opening at the top. Such bottles are produced from polyethylene terephthalate (PET) and are used in many cases for packaging for example water. It is known that the walls of the plastic packs are permeable in relation to low-molecular gases, and for that reason the periods of time for which it is possible to keep liquid foodstuffs, in particular juices and CO2-containing mixed drinks, are limited. In order to prevent the oxygen which is responsible for this from entering the pack or the carbon dioxide from escaping and in order in that way to make it possible to make better use of such plastic packs for liquid foodstuffs, manufacturers have occasionally gone over to coating the inside or the outside of the walls of those plastic packs. An example of such a known coating process for improving that so-called barrier property of plastic packs is plasma-enhanced coating from the gaseous phase (the English abbreviation used is PECVD). In that respect, in the case of the inner coating, a gas mixture is introduced into the pack or the hollow body. Electromagnetic energy is radiated thereinto, and fires the plasma, and in that treatment procedure, for example a vitreous silicon oxide or an amorphous carbon layer is applied in the interior of the hollow body. Low-molecular gas, for example oxygen or carbon dioxide, by virtue of that additional barrier, can penetrate through the plastic walls substantially less to not at all, and low-molecular organic compounds can also no longer penetrate into the plastic material, with the consequence that, in the case of packaged liquid foodstuffs, the drinks can thereby keep their taste and their quality for substantially longer. There are however various factors which involve a coating of different thicknesses on the wall surface, for example the differing configuration of the coated bodies.
In accordance with another known method, X-ray fluorescence is used to measure on a wall portion which is cut out of the bottle, whether an application has taken place. This method also suffers from the disadvantage that it is not possible to establish whether the application is homogenously distributed on the surface and the coating therefore enjoys a good barrier property. In addition this elementary analysis can only be established on a stamped-out portion of the wall of the pack.
Therefore a further method has been developed whereby the barrier property of a coating on a plastic wall is measured by means of the oxygen which diffuses through the wall and the coating. For example a pack which is in air is flushed in the interior with nitrogen for a period of one day until the interior of the pack is certain to contain almost no longer any oxygen, while outside the pack there is a gas with 21% oxygen. If, after that flushing operation with nitrogen, the internal atmosphere of the hermetically sealed pack is left at rest, then the oxygen which possibly accumulates there in the course of time originates from the atmosphere surrounding the pack. That oxygen has then migrated from the outside through the plastic wall inwardly. After a waiting time of at best about twenty-four hours (typically up to three weeks), this measuring method provides that a steady final value in respect of the oxygen content in the atmosphere in the interior of the pack has become established. If that value is compared to a predetermined barrier value, it is then possible to establish whether the measured pack has been coated with a sufficiently good barrier. There is the disadvantage here that this known checking method requires a very long period of time so that the method can scarcely be used in the course of pack production as real time measurement is practically impossible to implement.
Therefore the object of the invention is to provide a method of the kind set forth in the opening part of this specification, by means of which the barrier property of the coating can be measured in a relatively short time, reliably and possibly for the entire surface of the coating, without destroying the body or the pack having the plastic wall.
In accordance with the invention that object is attained in that:
firstly the plastic wall is subjected to the action of a fluid which contains a test medium with at least one component which can diffuse into the plastic wall and be absorbed there,
after the step of subjecting the coated space to the action of the fluid test medium deposited on the coated surface is removed therefrom,
thereafter the coated space is subjected to the action of an extraction medium in which the test medium is well soluble and the test medium can be measured, and
the concentration of the test medium in the extraction medium is measured.
This novel measuring method can provide for measuring the coating of the entire surface of a plastic wall, for example on the inside or the outside of a pack, which for example can be in bottle form, in regard to the barrier properties. It is therefore possible to measure whether low-molecular gases, preferably oxygen or carbon dioxide, can or cannot diffuse through the coating. If the coating is non-homogenous or if there are indeed places at which the wall of the pack is not coated at all, a markedly greater degree of diffusion of oxygen will be measured than if the coating is homogenous on the entire surface area in such a way that the oxygen cannot pass through the wall, without overcoming the barrier.
Both in the known method and also in the method according to the invention the space on the coating side, in the case of an internal coating therefore the internal space of the pack, is admittedly subjected to the action of a first fluid, for example flushed therewith, and is then flushed through until there is no longer any low-molecular gas in that space, which is intended to be prevented by the coating from passing through or diffusing through the plastic wall. In the known case the coated space is subjected to the action of nitrogen which is not a test medium. In accordance with the invention in contrast the first fluid contains a test medium for example acetaldehyde or hydrogen peroxide (H2O2). In the known method the flow of the low-molecular gas, for example oxygen, is always measured in one direction. For example the method involves measuring whether oxygen outside the plastic pack (in the air) diffuses through the plastic wall into the interior thereof, in spite of the coating. A reversed flow is neither produced nor measured. In accordance with the invention in contrast the coated space is admittedly also flushed with the first fluid containing the test medium, for example H2O2, but no account whatsoever is taken of the oxygen outside the pack. Rather, such a test medium is selected, which can diffuse into the plastic wall and also be absorbed there. If the coating were theoretically very good with a 100% barrier property, then no test medium would be absorbed in the coating and definitely not in the plastic wall, it could thereafter also not be extracted and consequently it could not be measured. The gas flow is therefore provided in two directions, more specifically for example from the interior of the pack in the plastic walls outwardly and thereafter back again out of those walls or the coating back into the internal space in the pack. The advantageous independence of low-molecular gases which are outside the pack is recognised, insofar as the internal space is coated (and vice-versa). It would in that respect moreover be possible to introduce a plurality of various test media into the first fluid.
Destruction of the plastic wall is avoided. There is no need for wall portions to be stamped out and measured separately. In spite of measurement of the entire hollow body the barrier property is checked in a relatively short period of time. Good reliable measurements have already been obtained in ten minutes or less. Measuring apparatuses for measuring the concentration of one medium in another are available in the art. It is possible to accurately and rapidly measure how much test medium, for example H2O2, is contained in an extraction medium, for example water or air.
Under other conditions it is possible for the total measurement procedure to be carried out within two hours, in which respect it has been found that this period of time is always still reasonable and useable for production monitoring. More specifically, in that way it is possible to increase in particular the level of measuring accuracy as the total amount of test medium extracted increases with an increasing extraction time.
The basic notion of the novel measuring method is that the attempt is made to introduce a well-chosen test medium from the coating side into the coating and/or the plastic wall therebehind (subjecting it to the action thereof) so that at least one diffusible and absorbable component can accumulate in the plastic wall and/or its coating, whereupon any test medium which has been deposited is removed again by means of a propellant, for example gas or liquid, from the surface at the coating side. Then, after that process of diffusing into the material, the test medium will try to diffuse out of same again. In accordance with the invention in the meantime the coated space is subjected to the action of an extraction medium, for example water or air. This must be matched to the test medium so that the test medium can dissolve in the extraction medium and can also be measured in the extraction medium. The test medium therefore issues from the plastic wall and the coating into the extraction medium and there affords a level of concentration which in accordance with a calibration curve permits information to be provided about the quality of the barrier property of the coating.
It is to be noted that preferably a 25% aqueous solution of H2O2 is used as the test medium for coated PET containers.
It is desirable in accordance with the invention if, prior to subjecting the coated space to the action of the first fluid, said space and the surface of the coating are heated in a conditioning phase to a temperature at which the material of the coated plastic wall does not deform. In the case of PET as the plastic material it is desirable to effect heating to less than 60xc2x0 C. In that respect it is advantageously sufficient if an internally coated pack is heated only on the inside by blowing with a suitably warmed air flow. That conditioning procedure expedites the measuring method because thereafter the test medium diffuses more quickly into the layers to be investigated.
For further expedition of the entire measuring method it is desirable if, in accordance with the invention, the operation of removing the test medium from the coated surface is effected by blowing on it with warm air. While that step of removing the test medium for typical PET bottles of a volume of about 0.5 is deemed to be sufficiently concluded in just 20 seconds, that blowing operation and thus the operation of removing the test medium can be prolonged by between 1 and 2 minutes if the test medium is partially deposited by condensation on the surface of the coating. The tendency to experience condensation can depend on the concentration of the relevant component of the test medium, for example the concentration of hydrogen peroxide in water. Blowing on the coated surface with warm air at any event makes it possible to remove all residual amounts of test medium from the surfaces which are acted upon. A further advantageous expedition in the measuring method and ensuring reliable results is achieved if in accordance with the invention the extraction medium is at a temperature of about 20xc2x0 C. The extraction time can even be further curtailed to a few seconds if the temperature of the extraction medium is increased to non-critical values of for example between 50xc2x0 C. and 60xc2x0 C. (non-critical in the sense of successful measurement without interference factors).
A further advantageous configuration of the invention provides that the test medium is an approximately 25% aqueous solution of H2O2 and, for acting on the space at the coating side, that test medium is blown in, in a predetermined metered amount with warm air for between 2 and 10 seconds, preferably for between 3 and 7 seconds, and particularly preferably for 5 seconds.
It is also possible for the test medium used to be CO2; while the extraction medium can be nitrogen or an inert gas, preferably helium or argon. The method according to the invention can also be carried into effect in relation to walls of polyethylene or PVC. The device for carrying out the above-described method has an intermittently movable conveyor, over which a plurality of treatment stations are arranged at a spacing one behind the other in the conveyor direction. In accordance with the invention packs which are open at one side are held with their opening upwardly on that conveyor and conveyed from one treatment station into the next. Then, in accordance with the invention, in each treatment station a fluid introduction inlet can be arranged above the opening of the pack and connected at the upstream side to a feed conduit. In the last treatment station moreover the opening of the pack can be sealingly connected to a fluid discharge conduit. The treatment can be considered to be conditioning, subjecting the pack to the action of the test medium (for example spraying in the aqueous H2O2 solution), drying, extracting and measuring. One or more treatment stations can be used for the individual treatments. Through the fluid introduction inlet, different gases and/or liquids can be introduced into the pack through the opening of the pack and possibly taken out of the pack. In addition in the last treatment station a fluid discharge conduit can be applied to the pack. In that way it is possible for the amount of previously introduced fluid, which is urged out or sucked out, to be removed from the pack in quantitatively measurable manner. By embracing the fluid introduction inlet, through which for example the test medium has been introduced centrally in the above-described manner, the fluid discharge conduit can provide that a desired metered amount of the previously introduced fluid, for example the extraction medium, is removed and fed to a measuring chamber.
A further configuration of the device according to the invention can provide that in each treatment station after the conditioning operation the opening of the pack can be sealingly connected to a fluid discharge conduit. If CO2 is used as the test medium, this means that all packs to be measured are sealed off in relation to an outside atmosphere in which traces of gas are or can be present, which under some circumstances can result in falsification of the measurement result.
Specifically it is of particular advantage if in accordance with the invention the following treatment stations are provided in the direction of conveying movement of the conveyor:
1. a first treatment station for conditioning the pack with air,
2. a second treatment station for subjecting the pack to the action of test medium,
3. at least one third treatment station for removal from the pack of the test medium which has been deposited at the surface and which has not diffused into the wall,
4. a fourth treatment station for introducing extraction medium into the pack, and
5. a fifth treatment station for measuring the concentration of the test medium in the extraction medium.
In order to make the removal of residual amounts of test medium from the surface which has been acted upon more intensive, it may be appropriate to use three treatment stations instead of one. The entire measuring method can then be carried out expeditiously even under those conditions under which the test medium has been deposited forming a film of fluid on the surface due to condensation. The dry air which is blown in can then be used in each of the three treatment stations for removal of the test medium.
The component of the test medium which can diffuse into the material of the plastic wall or the coating is to be present in the highest possible level of concentration. The concentration gradient should be a maximum in relation to the other constituents of the medium so that a sufficient amount of that component of the test medium can be incorporated in the coated plastic wall. The measurement is proportionally more accurate, the greater the amount of test medium which first diffuses into the wall in a short unit of time and is thereafter extracted.
The test medium must also be so stable that it does not break up until the end of the operation of subjecting the coated surfaces to the action thereof. At least the above-mentioned component of the test medium should not break down. Those conditions are advantageously fulfilled when using the 25% aqueous H2O2 solution.
The test medium should also be heatable to the above-mentioned temperatures which are used in the measuring method. In other words, the test medium must enjoy good temperature resistance over the entire meaningful range of parameters of the measuring method.
The measuring method is generally monitored and governed by persons. The test medium should therefore not be harmful to the health of the operating personnel. It is preferable if an operator can carry out all steps in the method without protective clothing.
Packs are frequently used in the field of foodstuffs, so that the test medium should not be in any way toxic. It must be provided that the test medium is compatible with foodstuffs, without any risk.
If the test medium or at least the above-mentioned component thereof is introduced into the pack in the gaseous phase, homogenous distribution thereof over the entire surface of the coated pack is also guaranteed. All regions of a coating can then be jointly measured by a measuring procedure.
The test medium must also be of such a nature in chemical and physical terms that it does not destroy the coating.
Similar conditions are also to be provided for the extraction medium for which for example water or air or inert gases can be used.
The extraction medium should also be harmless in regard to foodstuffs and stable without the risk of decomposing, it should not destroy the coating and it should make it possible for the test medium to transfer readily into the extraction medium.
The steps mentioned in accordance with the invention set forth a measuring method in which the essential conditions can be monitored, that is to say checked, and can also be observed, and which delivers reliable results in 10 minutes or less.
For the device for carrying out the measuring method, in particular the amounts of air, temperature, time and also the amount of test medium and also extraction medium can be reliably checked and maintained.